1. Field of the Invention
The invention relates to fermented food products containing probiotic strains, and their preparation process.
2. Description of the Related Art
The bifidobacteria belong to the dominant anaerobic flora in the colon. The main species present in the human colon are Bifidobacterium adolescentis, Bifidobacterium bifidum, Bifidobacterium longum ssp infantis, Bifidobocterium breve, Bifidobacterium longum. 
The bifidobacteria are probiotic bacteria of choice. Bacteria of the genus Bifidobacterium are used in numerous products currently on the market and are often added to dairy products already comprising the standard bacteria in yogurt (Streptococcus thermophilus and Lactobacillus bulgaricus).
The consumption of bifidobacteria is recognized as being beneficial in the process of re-establishing the normal bifidobacteria population in individuals having undergone antibiotics therapy. This consumption also seems to make it possible to reduce constipation, prevent diarrhoea and reduce the symptoms of lactose intolerance.
Probiotics are live bacteria. The use of these live bacteria in the manufacture of food products such as dairy products is tricky in particular with regard to the problem of survival of these bacteria in the product.
80% of the products currently on the market which contain bifidobacteria do not satisfy the criteria making it possible to maintain that they significantly improve the intestinal transit of the individuals consuming them. A daily intake of at least 108 to 109 viable cells has been recommended as the minimum dose making it possible to have a therapeutic effect (Silva A. M., Barbosa F. H., Duarte R., Vieira L. Q., Arantes R. M., Nicoli J. R., Effect of Bifidobacterium longum ingestion on experimental salmonellosis in mice, J. Appl. Microbiol. 97 (2004) 29-37). The required dose can be dependent on the probiotic strain used.
In the case of the production of a bioactive food product containing bifidobacteria the problem therefore arises of obtaining a sufficient population of these bacteria in the product and maintaining it during the “life” of the product without resorting to technical solutions capable of altering the organoleptic qualities of the product.
The problem of the numerical size of the population of probiotic strains in a fermented dairy product is a known problem (see in particular D. Roy, Technological aspects related to the use of bifidobacteria in dairy products, Lait 85 (2005) 39-56, INRA, EDP Sciences).
Several reasons for this problem have been suggested, including the reduction in the population during storage, the disturbed growth of these bacteria starting from a certain pH or quite simply the poor ability of these bifidobacteria to grow, in particular in milk.
It is known that the fructo-oligosaccharides, certain starches, certain sugars, glycerol and certain yeast extracts have significant bifidogenic effects. On the other hand oxygen is toxic to certain probiotic strains.
The use of cysteine or ascorbate as an oxygen scavenger has therefore been described (A review of oxygen toxicity in probiotic yogurts: influence on the survival of probiotic bacteria and protective techniques. Talwalkar & Kailasapathy; Comprehensive Reviews in Food Science and Food Safety, 3 (3) 117-124; 2004), without it however having been demonstrated that the use of these substances makes it possible to obtain and maintain populations of bifidobacteria at the desired levels during storage. Moreover, the potentially negative effect of the cysteine on the final properties of a yogurt has been noted.
Generally, the fermented food products having properties of relative maintenance of the populations of bifidobacteria during the preservation of said products, and which are described in the literature, do not generally have acceptable organoleptic properties, due to the fact in particular that substances such as yeast extract are present in a high concentration in the products.